Audio, video and other types of data may be transmitted through a variety of types of network according to many different protocols. For example, data can be transmitted through a collection of networks usually referred to as the “Internet” using protocols of the Internet protocol suite, such as Internet Protocol (IP) and User Datagram Protocol (UDP).
Data is often transmitted through the Internet addressed to a single user. However, it can be addressed to a group of users. This is known as “multicasting”.
One way of multicasting data is to use an IP datacasting network. Through such an IP-based broadcasting network, one or more service providers can supply different types of IP services including on-line newspapers, radio, television and download of music songs, videos, pictures, games and software. These IP services are organised into sessions, each session comprising one or more media streams in the form of audio, video and/or other types of data.
To determine when and where these sessions occur, users refer to an electronic service guide (ESG). One example used in DVB is an electronic program guide (EPG). The electronic service guide is usually divided up into parts and transmitted to users.
This approach, however, has several drawbacks. On the one hand, if any sessions are updated, then the user usually has to wait until a new version of the service guide has been received before they receive notification of updated sessions. On the other hand, few sessions are usually updated. Therefore, much of the data received by the user is superfluous. This is wasteful both in terms of processing power and electrical power, both of which tend to be in short supply in battery-powered mobile terminals.
The present invention seeks to provide an improved method of announcing sessions transmitted through a network.